file: /proc/sys/net/ipv4/tcp_invalid_ratelimit
variable: net.ipv4.tcp_invalid_ratelimit
Official reference

Limit the maximal rate for sending duplicate acknowledgments in response to incoming TCP packets that are for an existing connection but that are invalid due to any of these reasons:

(a) out-of-window sequence number, (b) out-of-window acknowledgment number, or (c) PAWS (Protection Against Wrapped Sequence numbers) check failure

This can help mitigate simple “ack loop” DoS attacks, wherein a buggy or malicious middlebox or man-in-the-middle can rewrite TCP header fields in manner that causes each endpoint to think that the other is sending invalid TCP segments, thus causing each side to send an unterminating stream of duplicate acknowledgments for invalid segments.

Using 0 disables rate-limiting of dupacks in response to invalid segments; otherwise this value specifies the minimal space between sending such dupacks, in milliseconds.

Default: 500 (milliseconds).